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Wireless Networks

Lecture 13

AMPS and ETACS

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AMPS system uses 7-cell reuse pattern with provision ofsectoring and cell splitting to increase system capacity.

After extensive tests, it was found that 30 KHz channel

requires s SIR of 18 dB.

The smallest reuse factor which satisfies this requirementusing 120 degree directional antenna is N = 7

ETACS: European Total Access Communication System

Identical to AMPS except scaled to 25 KHz as opposed to 30 KHz

Different format of mobile identification number (MIN) due to need of

accommodating different country codes in Europe as

opposed to area code in US

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AMPS Architecture

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System Overview

AMPS and ETACS both use FM and FDD for radio transmissionlike other 1G systems

In US,

transmissions from mobiles to BS (reverse link) use frequencies

between 824-849 MHz While BS transmits to mobiles (forward link) using frequencies

between 869-894 MHz

A separation of 45 MHz between forward and reverse channels is dueto use of inexpensive and highly selective duplexers in mobile units.

The control channel and blank-and-burst data streams aretransmitted at 10kbps in AMPS and 8kbps in ETACS

These wideband streams have max frequency deviation of +8KHz and +6.4 KHz for AMPS and ETACS

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Each BS has

one control channel transmitter that transmits onforward control channel (FCC)

One control channel receiver that listen to reversecontrol channel (RCC) to set-up a call

8 or more duplex voice channels

Commercial BS supports as many as 57 voice

channels

Forward Voice Channel (FVC) carry theconversation originating from landline caller

to cellular subscriber 6

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The actual number of control and voice channels varies widelydepending on the traffic, maturity of the system and locationof other BSs.

The number ofBS in a service area varies widely as well from

few towers in rural area to several hundred or more BS in alarge city.

Each BS continuously transmits digital FSK data on FCC at alltimes so that idle subscriber units can lock onto the strongestFCC.

All users must be locked onto a FCC in order to originate orreceive calls.

The BS RCC receiver constantly monitors transmission fromsubscribers that are locked onto the matching FCC

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In US AMPS, there are 21 control channels and ETACSsupports 42 control channels per provider

Thus any cellular phone needs to scan limited number ofcontrol channels to find best serving BS

It is upto the service providers to make sure adjacent FCC are

not assigned to nearby BSs The nonwireline service provider (A provider) is assigned

odd system identification number (SID) and wireline serviceprovider (B provider) is assigned even SID.

SID is transmitted once every 0.8 seconds on each FCC, along

with other overhead data which reports the status of cellularsystem

In ETACS area identification numbers (AID) are used instead ofSID.

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Call handling

Call: landline user cellular subscriber

From PSTN arrives at MSC.

A paging request is sent out with subscriber MIN

simultaneously on every BS FCC.

If intended subscriber receives its page on FCC, it

responds with ACK on RCC.

The MSC directs the BS to assign FVC and RVC pairto take place call

The BS also assigns supervisory audio tone (SAT)

and a voice mobile attenuation code (VMAC) as it

moves the call to the voice channels 9

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SAT

it allows user and BS to distinguish each other from co-channel users

located in different cells

Transmitted continuously on the both FVC and RVC at three different

frequencies 5070 Hz, 6000 Hz, 6030 Hz

VMAC

Instructs the user to transmit at a specific power level

Once on the voice channel, wideband FSK data is used by BS and

subscriber in a blank-and-burst mode to initiate handoffs, change

transmitter power as needed and provide other system data

Blank-and-burst signaling allows the MSC to send bursty data on voice

channels by temporarily omitting speech and SAT and replacing with

data.

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Call: mobile user landline user

Subscriber transmits request (MIN, electronic

serial number, station class mark and destination

number on RCC

If received correctly by BS, sent to MSC

MSC check if user is properly registered, connects

to the PSTN Assigns FVC and RVC with SAT and VMAC

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During a call, MSC issues numerous blank-and-

burst commands which switch

Between different voice channels on different BS

depending on where the user is traveling

The MSC uses scanning receiver called locator

in nearby BS to determine RSSI for handoff

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When a new call request arrives from PSTN or

subscriber

Voice channels may be occupied

MSN holds line open while instructing current BS

to issue directed retry to subscriber on FCC

It forces the subscriber to switch to different

control channel or BS depending on radiopropagation effects, current traffic, location of

subscriber

However it may or may not succeed.13

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AMPS and ETACS air interface

Parameter AMPS ETACS

Multiple Access FDMA FDMA

Duplexing FDD FDD

Channel BW 30 KHz 25 KHz

Traffic channels per RF channel 1 1

Reverse channel freq 824-849 MHz 890-915 MHz

Forward channel freq 869-894 MHz 935-960 MHZ

Voice modulation FM FM

Data rate on control/wideband channel 10kbps 8kbps

Spectral efficiency 0.33 bps/Hz 0.33 bps/Hz

Number of channels 832 1000

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Supervisory signals (SAT and ST

tones) Allow each user and BS to confirm that they are connected

during a call

SAT always exists during use of any voice channel.

AMPS and ETACS use three SAT signals at frequencies of 5970

Hz, 6000 Hz or 6030 Hz

BS constantly transmits one of three SAT tones on each voice

channels when in use

SAT is superimposed on voice signal on both forward and

reverse channels

The particular frequency of SAT denotes location of BS and is

assigned by MSC

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When a call is setup and a voice channel is

issued

SAT is transmitted immediately on FVC

Subscriber unit begins monitoring FVC, it must

detect, filter and demodulate SAT

Similarly it reproduces SAT on RVC

This is required to dedicate a voice channel

If SAT is not presented or improperly detected

within a one second interval, Both BS and

subscriber unit cease transmission16

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Signaling Tone (ST)

It is a 10 kbps data burst which signals call

termination by the subscriber

It is a special end-of-call message containing

alternating 1s and 0s sent on RVC for 200 ms

Unlike blank-and-burst messages which briefly

suspends SAT transmission, ST tone must be sentsimultaneously with SAT.

Alerts the system that user has deliberately

terminated the call as opposed to being dropped

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Wideband Blank-and-burst

Encoding

AMPS voice channels carry wideband (10

kbps) data streams for blank-and-burst

signaling

The wideband data stream is encoded using

Manchester coding

The advantage is that the energy of the

Manchester coded signal is concentrated at

the transmission rate frequency of 10 KHz and

little energy leaks into audio band below 4

KHz 18

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Narrowband AMPS (N-AMPS)

10 KHz channel: 3 times large number of usersand bandwidth

Uses same SAT, ST and blank-and-burst except

signaling was done by using sub-audible datastreams

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